A series of complex snail/schistosome interactions govern the intramolluscan development of the parasite. In large part, the outcome of this relationship is determined by the susceptibility of the snail to the schistosome, and the infectivity of the schistosome for the snail. Research in this proposal is focussed on elucidating (a) factors which control the ultimate level of cercarial production in susceptible snails, and (b) genetic determinants of the snail governing the outcome of the initial miracidial infection. This research will use genetically well- characterized Biomphalaria glabrata stocks and Schistosoma mansoni strains. Such well characterized populations have enabled investigators in the past to dissect many important events in the early parasite/snail interaction. A major part of the effort will be devoted to studying the ill- defined events controlling the level of cercariogenesis in snails with varying degrees of susceptibility. This will also include a study of cercariogenesis in snails exhibiting diapause behavior, which (having the ability to estivate) are important to field control efforts. Other experiments will be conducted which will explore the potential modulation of resistance and/or cercarial production as a result of schistosome antigenic influences within the snail. Through the use of gene probes we will also attempt to identify markers in snail DNA which correlate with resistance and other phenotypes. Through these efforts we should be able to increase our understanding of the genetic factors affecting the miracidium/snail encounter, and increase our understanding of the natural transmission of S. mansoni, which could lead to the development of effective field control measures in the future.